Whenever I make changes to DNS I have noticed that it takes hours to propagate when viewing from my wired connection but with minutes I can see the changes reflected when browsing on my mobile, 3g T-mobile network.

I really would like to understand why this happens.

With regards to the question being closed:
The question is fine. Closing it for not being "relevant to professional system administration" is both incredibly annoying and incorrect.

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1 Answer
1

DNS records have a TTL (time to live). This in seconds and can be anywhere from 0 to weeks.

dig www.google.com
www.google.com. 300 IN A 74.125.239.145

In this case the TTL is 300 seconds and a conforming server will only keep the record for 300 seconds before refreshing it. Unfortunately, DNS records get cached in many different places and not all software does the correct thing wrt honoring the TTL. Web browsers are notorious for this, but the worst offender I know of is the nscd daemon on linux systems.

It can often be quite difficult to figure out exactly which piece of software is holding on to out of date records.